Pulse shape discrimination (PSD) of organic scintillators involves subtle physical phenomena which give rise to the delayed luminescence characteristic of neutrons, providing a means of distinguishing neutrons from the preponderance of prompt luminescence arising from background gamma interactions. The mechanism by which this occurs begins with intersystem crossing (ISC), where the excited singlet state (SI) nonradiatively relaxes to the excited triplet (T), as shown in FIG. 1. In FIG. 1, the basic physical processes leading to the delayed fluorescence characteristic of neutron excitation of organics with phenyl groups is shown.
Since the triplet is known to be mobile in some compounds, the energy migrates until two triplets collide and experience an Auger upconversion process, shown as Equation 1:Ti+T1→S0+S1   Equation 1
In Equation 1, T1 is a triplet, S0 is the ground state, and S1 is a first excited state. Finally, the delayed singlet emission occurs with a decay rate characteristic of the migration rate and concentration of the triplet population, which is represented as Equation 2:S1→S0+hv   Equation 2
In Equation 2, hv is fluorescence, while S0 is the ground state and S1 is a first excited state. The enhanced level of delayed emission for neutrons arises from the short range of the energetic protons produced from neutron collisions (thereby yielding a high concentration of triplets), compared to the longer range of the electrons from the gamma interactions. The resulting higher concentration of triplets from neutrons, compared to gamma interactions, leads to the functionality of PSD. The observation of PSD is believed to be in part related to the benzene ring structure, allowing for the migration of triplet energy.
It is generally accepted in the prior art that stilbene offers good PSD. However, stilbene is notoriously difficult to obtain due in part to its standard formation by melt growth. It is presently believed that the expensive melt growth process is a superior process able to create large enough crystals for use as large-crystal scintillators, e.g., in single-crystal radiation detectors, etc. The limited availability of stilbene and other melt grown crystals coupled with their high cost presents a serious challenge to radiation detector development.
Accordingly, it would be beneficial to grow organic scintillator crystals, including stilbene and other types of crystals, using solution growth, which should thereby increase access to such materials as well as significantly reduce their cost.